The present invention is directed toward treatment of tachyarrhythmias, which are excessively fast heart rhythms. In particular, the present invention is directed toward treatment of tachycardias.
Therapies have been developed for treating tachycardias by destroying cardiac tissue containing identified ectopic foci or aberrant conduction pathways. A variety of approaches have been taken, including application of electrical energy or other forms of energy to destroy the undesired cardiac tissue. As examples, ablation of cardiac tissue has been accomplished by means of radio frequency electrical current, microwave energy, heat, electrical pulses, cryothermy, and lasers. At present, ablation using R-F energy is perhaps the most widely practiced in the context of ablation procedures that can be carried out by means of a catheter, inserted into the closed heart.
Most R-F ablation catheters employ electrodes which are intended to contact the endocardium or, in some cases as in U.S. Pat. No. 5,083,565, are intended to penetrate the endocardium, and enter the myocardium. In general, R-F ablation catheters are effective to induce small lesions in heart tissue including the endocardium and inner layers of myocardium, in the immediate vicinity of the electrode.
R-F ablation causes tissue in contact with the electrode to heat through resistance of the tissue to the induced electrical current therethrough. Reliably sensing, however, when the electrode is in actual contact with the heart tissue so that the ablation procedure may begin is required. Many schemes for sensing electrode contact with the tissue have been proposed. For example, ablation systems of Biosense, Inc. detect wall contact through the stability of the sensed EKG. Such an approach is not completely satisfactory. In patients who have an infarction, for example, the tissue near the infarction often cannot provide an acceptable EKG signal. Ablating near the infarcted area, however, is often times the specific area in which ablation is needed to be performed. Still further, the stability of the EKG actually only indicates the stability of contact during the 50-100 ms in which the P or QRS complex is present. The electrode may still bounce against the moving heart wall and still provide a stable EKG signal. Other ablation systems, such as those sold by the CarioRythm division of Medtronic Inc., detect wall contact through impedance. This approach is also not completely satisfactory. Displacement or movement within the heart of the ablation catheter relative to the indifferent skin electrode alters the impedance detected, thus distorting the detection of wall contact. Moreover, such measurements may also vary from patient to patient. Finally, the variations in impedance due to wall contact are often too small to be a reliable indicator of actual wall contact. Thus there exists a need for and ablation system which permits the electrode tissue contact to be reliably indicated.